Process for raising the melting point of poly-e-caprolactam filaments, threads, and yarns by treatment with gaseous formaldehyde and products produced thereby



March 26, 1957 GABLER 2,786,732

PROCESS FOR RAISING THE MELTING POINT OF POLY-E-CAPROLACTAM FILAMENTS, THREADS AND YARNS BY TREATMENT WITH GASEOUS FORMALDEHYDE AND PRODUCTS PRODUCED THEREBY Filed March 5, 1955 INVENTOR. 91/004; 645451? PROCESS FOR RAISING THE MELTING POINT OF PULY-e-CAPROLACTAM FILAMENTS, THREADS, AND YARNS BY TREATMENT WITH GASEOUS FORMALDEHYDE AND PRODUCTS PRODUCED THEREBY Rudolf Gabler, Tamins, Switzerland, assignor to Iuveuta A. G. fuer Forschung und Patentverwertung, Zurich, Switzerland Application March 3, 1953, Serial No. 340,051

Claims priority, application Switzerland August 20, 1049 6 Claims. (Cl. 8-1155) This invention relates to the production of products with high melting points from polyamides. More par ticularly, the invention relates to a process for improving the properties of shaped articles prepared from e-caprolactam. Among these shaped articles are threads and yarns for general use, and cord as well as cord fabrics for tires. This application is a continuation-in-part to the co-pending applications Ser. Nos. 176,052 and 176,053 both filed July 26, 1950, now abandoned.

The improvement in the properties of the above mentioned articles is brought about, generally speaking, by a subsequent treatment of the polyamides with aldehydes.

Attempts have already been made to, improve the properties of fibers, threads and other articles made therefrom, which were derived from synthetic linear polyamides, by subsequent treatment of the same with formaldehyde. Treatments of this kind have been carried out either by dissolving the polyamide and heating it with formaldehyde; or by treating the shaped polyamide with formaldehyde in a soaking liquid, such as water or alcohol in the presence of an acid catalyst. By this treatment, the formaldehyde is attached to the nitrogen atoms or polyamide chains in the form of methylol groups. The methylol polyamides obtained by evaporation of the solvent or soaking means are easily soluble products having, as a rule, lower melting points than the starting material.

It has also been discovered that, by heating the methylol compounds, interlocked insoluble products may result, whose melting point is considerably above that of the starting material. It may be. assumed that in these interlocked polyamides the neighboring molecular chains are connected by CH2 bridges.

These products are, however, not satisfactory in the further processing. If the polyamides undergo formaldehyding treatment in solution, the resulting products cannot be spun readily. Even when formaldehyding is carried out in the presence of soaking means, the structure of the polyamides is so extensively altered that while the treated product is distinguished by a high melting point and improved elasticity, it has completely lost its other typical polyamide characteristics, in particular the high tensile strength.

This is also true of another known method which con sists in heating undrawn filaments and other articles derived from nylon with an alcohol and with formaldehyde vapors in the presence of an acidic catalyst. While this process is aimed at and actually yields articles of high elasticity, it is not useful for the production of fibers, threads and the like exhibiting high tensile strength.

The reduction in strength is probably due to the looseningo'f-the structure by the soaking means and, furthermore, the space required by the methylol groups formed in the first reaction step has alooseni'ng efiect and a partial demolition of the polyamide chains finally seems to take place in the acid medium.

nited States Patent F Certain disadvantageous properties of polyamide fabrics, which are hardly disturbing in other applications, as for example the always present plastic stretch, a certain thermoplasticity at temperatures such as occur in highly stressed tires, as well as their low afiinity for rubber or rubber-like coverings hitherto still stood in the way of general use of polyamide cord fabrics.

There has been no lack of attempts to eliminate these.

disadvantages and patent literature contains a series of processes suitable for removing one or the other of the deficiencies mentioned.

Thus, for instance, it was possible to improve the affinity for rubber of the linear polyamide, such as nylon, by treating it with diisocyanates. This treatment, however, did not effect an improvement with respect to plastic stretch and the thermoplastic range. Other treatments were likewise not fully successful.

The inventor has discovered that the above-mentioned disadvantages of the formaldehyding and other treatments used hitherto can be completely avoided if the products undergoing the treatment are first subjected to a cold drawing operation and thereafter treated with gaseous formaldehyde in the absence of all means adapted to dissolve or soak the treated material. Since the formation of methylol bridges takes place with simultaneous removal of water, this reaction is accelerated when working in the absence of water. Obviously, the acceleration is so effective that the intermediary alkylol stage is of short duration and the formation of methylol bridges is achieved under essential maintenance of the original crystalline structure.

As a rule, the materials to be treated are only subjected to a cold-drawing process, i. e. a drawing process occurring-at room temperature, in a single stage. In some cases, for instance for cord yarns, it is advantageous to operate in two stages, carrying out a hot-drawing operation subsequently to the cold-drawing.

The cold-drawing operation brings about an increase in the length of the fibers by about 250-400%; hot-drawing at temperatures of about 180 C. effects a further increase in length of 2050%.

The drawing operation may be carried out in any known manner. The yarns may, e. g. be wound onto perforated metal cylinders, while being drawn. If a twisting operation is to be combined with the drawing operation, i. e. if thread formation is intended, the fibers are rewound from bobbins onto perforated metal cylinders, the tensioning force on the thread being at least 1 gram/ den.

On the perforated metal cylinders, the threads are wound with a large angle of crossing, e. g. an angle of 3045. This is done for the purpose of making the inner layers of thread easily accessible to the agents with which they are subsequently treated; the washing liquids during the cleansing stage, dry .air during the drying step, and vapors of aldehyde during the aldehyde operation. The three operations could also be carried out subsequently in the same apparatus (see drawing).

The temperature range for the subsequent formaldehyding with gaseous formaldehyde lies between and 180 C., preferably at C. I may proceed by letting the formaldehyde vapors, which are developed from paraformaldehyde or some other material furnishing formaldehyde, react with the polyamide by means of a heated inert gas, for example nitrogen. The reaction may be considerably accelerated by the use of pressure, but may also be carried out at atmospheric pressure.

After such treatment, these threads are outwardly unchanged, but their melting point has been raised to 330 Patented Mar. 26, 1957'- What I claim is:

1. The process for raising the melting point of articles consisting of polye-caprolactam synthetic linear polyamide fibers, which comprises cold-drawing said fibers to about 350%-400% of their original length, drying the same to free them completely from water, and thereafter reacting them with dry gaseous formaldehyde, at a temperature ranging from 120 to 180 C. in an environment substantially consisting of an inert gas, and being free from catalysts, for a period of several hours, until the weight of the treated material has increased by about -14%.

,2. The process as set forth in claim 1, wherein the reaction of the articles with the gaseous formaldehyde is performed in a stream of nitrogen.

3. Poly-e-caprolactam synthetic linear polyamide filaments, threads, and yarns, produced by subjecting said filaments, threads and yarns to a cold-drawing process, whereby they are extended to from 350%-400% of their original length, then drying the same to free them completely from water, and thereafter reacting them with dry gaseous formaldehyde, at a temperature ranging from l20 to 180 C. in an environment free from catalysts and substantially consisting of an inert gas for a period of several hours, said treated material having a weight increase, as compared to the starting material, of from 8- 14%, and a melting point raised by from to C.

4. A cord fabric for vehicle and airplane tires made of poly-e-caprolactam synthetic linear polyamide fibers produced by subjecting said polyamides to a cold-drawing process whereby they are extended to 350%-400% of their original length, dried, and thereafter reacted with dry gaseous formaldehyde, at a temperature ranging from 120 to 180 C. in an environment free from catalysts and substantially consisting of an inert gas for a period of several hours and having as a result of said reaction a weight increase, as compared to the starting material, of about 8-14% and a melting point increased by 80-150" C. 5. The process of improving poly-e-caprolactam synthetic linear polyamide fibers to be made into cord fabric, which comprises cold-drawing said fibers to from 350% to 400% of their original length, drying the same to free them completely from water, and thereafter reacting them with dry gaseous formaldehyde, at a temperature ranging from 120 to 180 C. in an environment free from catalysts and substantially consisting of an inert gas for a period of several hours until the weight of the treated material has increased by about 8-15 6. The process of improving poly-e-caprolactam synthetic linear polyamide fibers to be made into cord fabric, which comprises subjecting said fibers to a colddrawing operation at room temperature until an extension of the fiber length of about 250400% has been reached, then subjecting the fibers to a hotdrawing operation at a temperature of about -190 C. until a further extension of the fibers of about 20 to 50% is obtained, drying the same to free them completely from water, and thereafter reacting them with dry gaseous formaldehyde, at a temperature ranging from 120 to C. in an environment free from catalysts and substantially consisting of an inert gas for a period of several hours until the weight of the treated material has increased by about 8-15%.

References Cited in the file of this patent UNITED STATES PATENTS 2,430,953 Schneider Nov. 18, 1947 2,434,247 Lewis et a1 Ian. 13, 1948 2,516,562 Graham July 25, 1950 2,540,726 Graham et al. Feb. 6, 1951 FOREIGN PATENTS 582,520 Great Britain Nov. 19, 1946 OTHER REFERENCES Hall: Silk Journal and Rayon World, April 1947, pages 30-32. 

3. POLY-E-CAPROLACTAM SYNTHETIC LINEAR POLYAMIDE FILAMENTS THREADS, AND YARNS, PRODUCED BY SUBJECTING SAID FILAMENTS, THREADS AND YARNS TO A COLD-DRAWING PROCESS, WHEREBY THEY ARE EXTENDED TO FROM 350%-400% OF THEIR ORIGINAL LENGHT, THEN DRYING THE SAME TO FREE THEM COMPLETELY FROM WATER, AND THEREAFTER REACTING THEM WITH DRY GASEOUS FORMALDEHYDE, AT A TEMPERATURE RANGING FROM 